1. Field of Invention
This invention relates to apparatus and methods for reducing noise audible from a speaker and more particularly for reducing such noise when no audio program content is imminent.
2. Background of the Invention
Audio signals in audio devices are typically produced and manipulated at zero decibels above one milliwatt (0dBm) which is a line level signal having a voltage level of about 0.775 Volts. These audio signals are manipulated in audio equipment such as signal processing equipment and are usually ultimately amplified to some greater voltage level at high drive current levels to provide sufficient power to drive a speaker. The power required to drive a speaker is considerably greater than the 0dbm signals manipulated in signal processing equipment and therefore high gain amplifiers are used to amplify audio signals up to speaker drive levels. These high gain amplifiers however, often have a DC bias point at their input, which establishes a common mode voltage enabling an audio signal having positive and negative signal swings to be received at the input. This common mode voltage results in a quiescent current flowing into the amplifier and this current acts as an input signal, which is amplified by the high gain amplifier causing an amplified quiescent current signal to be present in the drive signal provided to the speaker. This amplified quiescent signal appears as loud white noise in some systems.
Attenuating the quiescent current by using low noise amplifier designs is desirable, but still does not sufficiently reduce the audibility of the amplified quiescent signal when high volume levels are sought from the amplifier. This effect is quite noticeable on speaker systems used with personal computers.
What would be desirable therefore is a system which reduces or eliminates the audibility of the amplified quiescent current signal, especially in personal computer systems.